Laser-based surface acoustic wave dispersion spectroscopy for extraction of thicknesses, depth, and elastic parameters of a subsurface layer: Feasibility study on intermetallic layer structure in integrated circuit solder joint

Abstract

Laser beam deflection in combination with optical heterodyne diffraction is used to detect surface acoustic waves that are generated by impulsive laser light on a Sn–CuxSny–Cu–Si multilayer structure with layer thicknesses of the order of 1 μm. The acoustic phase velocity dispersion curves of the lowest two-surface acoustic wave modes are determined by spectral analysis of the experimental signals. The sensitivity of the dispersive behavior to the thickness and elastic properties of the individual layers is analyzed on the basis of the experimental data and of simulated data with noise added. The elastic parameters of the CuxSny intermetallic alloy are determined. Statistical least squares and most squares uncertainties on all best fitting material parameters are determined, giving a quantitative measure of the feasibility of parameter extraction by this method. The feasibility of solving the inverse problem of elastic depth profiling of a multilayer by the proposed wideband and multimode SAW dispersion spectroscopy method is confirmed.